[libc] Add a convenience CMake rule to add testsuites.

[libc] Add a convenience CMake rule to add testsuites.

Summary:
This rule helps avoid repeated setting of check-libc's dependency on the
various testsuites.

Reviewers: abrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D72353